This invention relates to a pressure sensor and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations.
Capacitive pressure sensors are well known in the prior art. Such sensors typically have a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A deformable, conductive member, such as a metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Since the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance of the sensor inversely proportional to the gap, d, between the central portion of the diaphragm and the conductive surface of the fixed element. In order for there to permit a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.
In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations in the sensor capacitor. For relatively high sensitivity, such sensors require high changes of capacitance in response to relatively small gap changes. To achieve such sensitivity from unit to unit, nominal gap dimensions and plate parallelism must be well controlled. Consequently, such sensors require that their component parts be manufactured to very close tolerances to establish the required dimensional relationships. In addition, the structure and materials must maintain those relationships over a useful temperature range.
In one form of prior art sensor, exemplified by the sensor of the Model 237 transducer manufactured by Setra Systems, Inc., the fixed metallic electrode is supported with respect to the diaphragm support member by means of an electrically non-conductive, high temperature glass portion. Because of the differences in the thermal expansion coefficients between such glass and the metal electrode and diaphragm support member, temperature changes cause changes in the gap between the fixed electrode and diaphragm (i.e. the plates of the capacitor), resulting in erroneous pressure readings. In addition, the effective seal between the regions on opposite sides of the diaphragm may only be economically maintained over a relatively small temperature range. Thus, such sensors give reliable pressure readings over only a relatively small range of temperatures.
Moreover, during the manufacture of such sensors, the fixed electrode must be set in the glass portion and then the conductive plate portion must generally be tailored (e.g. by lapping) to establish the necessary gap and parallelism. Consequently, those variable capacitance pressure sensors are relatively expensive to manufacture.
Moreover, during the manufacture of such sensors, the fixed electrode is generally set into the glass portion while the glass is in its molten state. As the assembly cools, mechanical stresses are set up which typically alter the desired initial gap dimension or degrade the parallelism between the capacitive plates. Following cooling of the assembly, the fixed electrode may need to be tailored (e.g. by lapping) to re-establish the critical gap and parallelism. In view of these processing steps, such sensors are relatively difficult and corespondingly expensive to manufacture.
It is, therefore, an object of this invention to provide a capacitive pressure sensor which overcomes the disadvantages known in the prior art. Specifically, an object of this invention is to provide such a pressure sensor which maintains relatively high sensitivity over a large temperature range.
Another object of this invention is to provide a capacitive pressure sensor which is highly reliable, inexpensive and easy to manufacture.